diff --git a/crates/decodal-core/src/diagnostic.rs b/crates/decodal-core/src/diagnostic.rs
index 2caf4c2..ef95599 100644
--- a/crates/decodal-core/src/diagnostic.rs
+++ b/crates/decodal-core/src/diagnostic.rs
@@ -10,6 +10,7 @@ pub struct Diagnostic {
     pub span: Span,
     pub message: String,
     pub labels: Vec<DiagnosticLabel>,
+    pub notes: Vec<String>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -25,6 +26,7 @@ impl Diagnostic {
             span,
             message: message.into(),
             labels: Vec::new(),
+            notes: Vec::new(),
         }
     }
 
@@ -39,6 +41,11 @@ impl Diagnostic {
         });
         self
     }
+
+    pub fn with_note(mut self, message: impl Into<String>) -> Self {
+        self.notes.push(message.into());
+        self
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
diff --git a/crates/decodal-core/src/eval.rs b/crates/decodal-core/src/eval.rs
index 13dc5d5..aadd4b1 100644
--- a/crates/decodal-core/src/eval.rs
+++ b/crates/decodal-core/src/eval.rs
@@ -21,6 +21,7 @@ pub struct Engine<L = EmptyLoader> {
     modules: Vec<Module>,
     imported_values: Vec<ImportedValue>,
     thunks: Vec<Thunk>,
+    thunk_import_origins: Vec<Option<ImportedValueOrigin>>,
     envs: Vec<Env>,
 }
 
@@ -29,6 +30,39 @@ struct ImportedValue {
     root: ThunkId,
 }
 
+#[derive(Debug, Clone)]
+struct ImportedValueOrigin {
+    key: String,
+    path: Vec<String>,
+}
+
+impl ImportedValueOrigin {
+    fn root(key: String) -> Self {
+        Self {
+            key,
+            path: Vec::new(),
+        }
+    }
+
+    fn field(&self, name: String) -> Self {
+        let mut path = self.path.clone();
+        path.push(name);
+        Self {
+            key: self.key.clone(),
+            path,
+        }
+    }
+
+    fn index(&self, index: usize) -> Self {
+        let mut path = self.path.clone();
+        path.push(format!("[{index}]"));
+        Self {
+            key: self.key.clone(),
+            path,
+        }
+    }
+}
+
 impl Engine<EmptyLoader> {
     pub fn from_parse(ast: Ast, root: ExprId) -> Self {
         let mut this = Self::new(EmptyLoader);
@@ -52,6 +86,7 @@ impl<L: ImportLoader> Engine<L> {
             modules: Vec::new(),
             imported_values: Vec::new(),
             thunks: Vec::new(),
+            thunk_import_origins: Vec::new(),
             envs: vec![Env {
                 parent: None,
                 bindings: Vec::new(),
@@ -261,8 +296,9 @@ impl<L: ImportLoader> Engine<L> {
                 if let Some(value) = self.find_imported_value(&key) {
                     return self.force(value);
                 }
-                let value = self.internalize_host_value(value)?;
-                let root = self.add_value_thunk(value);
+                let origin = ImportedValueOrigin::root(key.clone());
+                let value = self.internalize_host_value_with_origin(value, Some(&origin))?;
+                let root = self.add_value_thunk_with_import_origin(value, origin);
                 self.imported_values.push(ImportedValue { key, root });
                 self.force(root)
             }
@@ -836,17 +872,59 @@ impl<L: ImportLoader> Engine<L> {
                 .iter()
                 .position(|lhs_field| lhs_field.name == rhs_field.name)
             {
+                let lhs_thunk = lhs.fields[index].value;
                 let lhs_field_span = lhs.fields[index].span;
-                let lhs_value = self.force(lhs.fields[index].value)?;
+                let lhs_origin = self.thunk_import_origin(lhs_thunk).cloned();
+                let rhs_origin = self.thunk_import_origin(rhs_field.value).cloned();
+                let lhs_value = self.force(lhs_thunk)?;
                 let rhs_value = self.force(rhs_field.value)?;
                 let value = self
-                    .compose_and(lhs_value, rhs_value, span)
+                    .compose_and(lhs_value.clone(), rhs_value.clone(), span)
                     .map_err(|diag| {
-                        diag.with_label(span, format!("while composing field `{}`", rhs_field.name))
-                            .with_label(lhs_field_span, format!("left field `{}`", rhs_field.name))
-                            .with_label(rhs_field.span, format!("right field `{}`", rhs_field.name))
+                        let mut diagnostic = diag.with_label(
+                            span,
+                            format!("while composing field `{}`", rhs_field.name),
+                        );
+                        if lhs_origin.is_some() {
+                            diagnostic = annotate_imported_value(
+                                diagnostic,
+                                lhs_origin.as_ref(),
+                                Some(&lhs_value),
+                            );
+                        } else {
+                            diagnostic = diagnostic.with_label(
+                                lhs_field_span,
+                                format!("left field `{}`", rhs_field.name),
+                            );
+                        }
+                        if rhs_origin.is_some() {
+                            diagnostic = annotate_imported_value(
+                                diagnostic,
+                                rhs_origin.as_ref(),
+                                Some(&rhs_value),
+                            );
+                        } else {
+                            diagnostic = diagnostic.with_label(
+                                rhs_field.span,
+                                format!("right field `{}`", rhs_field.name),
+                            );
+                        }
+                        diagnostic
                     })?;
-                lhs.fields[index].value = self.add_value_thunk_with_span(value, rhs_field.span);
+                let result_span = match (lhs_origin.is_some(), rhs_origin.is_some()) {
+                    (false, true) => lhs_field_span,
+                    (true, false) => rhs_field.span,
+                    _ => rhs_field.span,
+                };
+                let result_origin = rhs_origin.clone().or(lhs_origin.clone());
+                lhs.fields[index].value = self.add_value_thunk_with_span_and_import_origin(
+                    value,
+                    result_span,
+                    result_origin,
+                );
+                if lhs_origin.is_some() && rhs_origin.is_none() {
+                    lhs.fields[index].span = rhs_field.span;
+                }
             } else {
                 lhs.fields.push(rhs_field);
             }
@@ -872,9 +950,14 @@ impl<L: ImportLoader> Engine<L> {
                 .position(|lhs_field| lhs_field.name == rhs_field.name)
             {
                 let lhs_value = self.force(lhs.fields[index].value)?;
+                let rhs_origin = self.thunk_import_origin(rhs_field.value).cloned();
                 let rhs_value = self.force(rhs_field.value)?;
                 let value = self.patch(lhs_value, rhs_value)?;
-                lhs.fields[index].value = self.add_value_thunk_with_span(value, rhs_field.span);
+                lhs.fields[index].value = self.add_value_thunk_with_span_and_import_origin(
+                    value,
+                    rhs_field.span,
+                    rhs_origin,
+                );
                 lhs.fields[index].span = rhs_field.span;
             } else {
                 lhs.fields.push(rhs_field);
@@ -924,38 +1007,68 @@ impl<L: ImportLoader> Engine<L> {
                         constraint,
                         span,
                         value_label,
-                        "value does not satisfy primitive type constraint",
+                        format!(
+                            "expected {primitive:?}, found {}",
+                            runtime_value_kind(&value)
+                        ),
                     ))
                 }
             }
             Constraint::ArrayItems(item_constraint) => {
+                let value_kind = runtime_value_kind(&value);
                 let RuntimeValue::Concrete(ConcreteValue::Array(items)) = value else {
                     return Err(self.constraint_violation(
                         constraint,
                         span,
                         value_label,
-                        "value does not satisfy array element constraint",
+                        format!("expected Array, found {value_kind}"),
                     ));
                 };
                 let item_constraint_value = self.force(*item_constraint)?;
                 let mut constrained_items = Vec::with_capacity(items.len());
                 for (index, item) in items.into_iter().enumerate() {
                     let item_span = self.thunk_span(item);
+                    let item_origin = self.thunk_import_origin(item).cloned();
                     let item_value = self.force(item)?;
+                    let composition_span = if item_origin.is_some() {
+                        constraint.span
+                    } else {
+                        item_span
+                    };
                     let constrained = self
-                        .compose_and(item_constraint_value.clone(), item_value, item_span)
+                        .compose_and(
+                            item_constraint_value.clone(),
+                            item_value.clone(),
+                            composition_span,
+                        )
                         .map_err(|diagnostic| {
-                            diagnostic
-                                .with_label(
-                                    constraint.span,
-                                    "array element constraint declared here",
+                            let diagnostic = diagnostic.with_label(
+                                constraint.span,
+                                "array element constraint declared here",
+                            );
+                            if item_origin.is_some() {
+                                annotate_imported_value(
+                                    diagnostic,
+                                    item_origin.as_ref(),
+                                    Some(&item_value),
                                 )
-                                .with_label(
+                            } else {
+                                diagnostic.with_label(
                                     item_span,
                                     format!("array element [{index}] checked here"),
                                 )
+                            }
                         })?;
-                    constrained_items.push(self.add_value_thunk_with_span(constrained, item_span));
+                    let constrained_span = if item_origin.is_some() {
+                        constraint.span
+                    } else {
+                        item_span
+                    };
+                    constrained_items.push(self.add_value_thunk_with_span_and_import_origin(
+                        constrained,
+                        constrained_span,
+                        item_origin,
+                    ));
                 }
                 Ok(RuntimeValue::Concrete(ConcreteValue::Array(
                     constrained_items,
@@ -996,7 +1109,7 @@ impl<L: ImportLoader> Engine<L> {
         constraint: &ConstraintEntry,
         operation_span: Span,
         value_label: Option<(Span, &'static str)>,
-        message: &'static str,
+        message: impl Into<String>,
     ) -> Diagnostic {
         let mut diagnostic = Diagnostic::new(
             DiagnosticKind::ConstraintViolation,
@@ -1032,6 +1145,14 @@ impl<L: ImportLoader> Engine<L> {
     }
 
     fn internalize_host_value(&mut self, value: HostValue) -> Result<RuntimeValue> {
+        self.internalize_host_value_with_origin(value, None)
+    }
+
+    fn internalize_host_value_with_origin(
+        &mut self,
+        value: HostValue,
+        origin: Option<&ImportedValueOrigin>,
+    ) -> Result<RuntimeValue> {
         match value {
             HostValue::String(value) => Ok(RuntimeValue::Concrete(ConcreteValue::String(value))),
             HostValue::Int(value) => Ok(RuntimeValue::Concrete(ConcreteValue::Int(value))),
@@ -1039,9 +1160,12 @@ impl<L: ImportLoader> Engine<L> {
             HostValue::Bool(value) => Ok(RuntimeValue::Concrete(ConcreteValue::Bool(value))),
             HostValue::Array(items) => {
                 let mut thunks = Vec::new();
-                for item in items {
-                    let value = self.internalize_host_value(item)?;
-                    thunks.push(self.add_value_thunk(value));
+                for (index, item) in items.into_iter().enumerate() {
+                    let item_origin = origin.map(|origin| origin.index(index));
+                    let value =
+                        self.internalize_host_value_with_origin(item, item_origin.as_ref())?;
+                    thunks
+                        .push(self.add_value_thunk_with_optional_import_origin(value, item_origin));
                 }
                 Ok(RuntimeValue::Concrete(ConcreteValue::Array(thunks)))
             }
@@ -1050,11 +1174,11 @@ impl<L: ImportLoader> Engine<L> {
                 mut constraints,
                 default,
             } => {
-                let item = self.internalize_host_value(*item)?;
-                let item = self.add_value_thunk(item);
+                let item = self.internalize_host_value_with_origin(*item, origin)?;
+                let item = self.add_value_thunk_with_optional_import_origin(item, origin.cloned());
                 let default = if let Some(default) = default {
-                    let value = self.internalize_host_value(*default)?;
-                    Some(self.add_value_thunk(value))
+                    let value = self.internalize_host_value_with_origin(*default, origin)?;
+                    Some(self.add_value_thunk_with_optional_import_origin(value, origin.cloned()))
                 } else {
                     None
                 };
@@ -1075,19 +1199,27 @@ impl<L: ImportLoader> Engine<L> {
             HostValue::Object(fields) => {
                 let mut object = ObjectValue { fields: Vec::new() };
                 for field in fields {
+                    let field_origin = origin.map(|origin| origin.field(field.name.clone()));
                     if object
                         .fields
                         .iter()
                         .any(|existing| existing.name == field.name)
                     {
-                        return Err(Diagnostic::new(
+                        let diagnostic = Diagnostic::new(
                             DiagnosticKind::Conflict,
                             Span::default(),
                             format!("duplicate host object field `{}`", field.name),
+                        );
+                        return Err(annotate_imported_value(
+                            diagnostic,
+                            field_origin.as_ref(),
+                            None,
                         ));
                     }
-                    let value = self.internalize_host_value(field.value)?;
-                    let value = self.add_value_thunk(value);
+                    let value = self
+                        .internalize_host_value_with_origin(field.value, field_origin.as_ref())?;
+                    let value =
+                        self.add_value_thunk_with_optional_import_origin(value, field_origin);
                     object.fields.push(ObjectField {
                         name: field.name,
                         value,
@@ -1101,8 +1233,8 @@ impl<L: ImportLoader> Engine<L> {
                 default,
             } => {
                 let default = if let Some(default) = default {
-                    let value = self.internalize_host_value(*default)?;
-                    Some(self.add_value_thunk(value))
+                    let value = self.internalize_host_value_with_origin(*default, origin)?;
+                    Some(self.add_value_thunk_with_optional_import_origin(value, origin.cloned()))
                 } else {
                     None
                 };
@@ -1209,13 +1341,48 @@ impl<L: ImportLoader> Engine<L> {
         self.add_thunk(ThunkKind::Value(value), span)
     }
 
+    fn add_value_thunk_with_import_origin(
+        &mut self,
+        value: RuntimeValue,
+        origin: ImportedValueOrigin,
+    ) -> ThunkId {
+        self.add_value_thunk_with_optional_import_origin(value, Some(origin))
+    }
+
+    fn add_value_thunk_with_optional_import_origin(
+        &mut self,
+        value: RuntimeValue,
+        origin: Option<ImportedValueOrigin>,
+    ) -> ThunkId {
+        self.add_value_thunk_with_span_and_import_origin(value, Span::default(), origin)
+    }
+
+    fn add_value_thunk_with_span_and_import_origin(
+        &mut self,
+        value: RuntimeValue,
+        span: Span,
+        origin: Option<ImportedValueOrigin>,
+    ) -> ThunkId {
+        self.add_thunk_with_import_origin(ThunkKind::Value(value), span, origin)
+    }
+
     fn add_thunk(&mut self, kind: ThunkKind, span: Span) -> ThunkId {
+        self.add_thunk_with_import_origin(kind, span, None)
+    }
+
+    fn add_thunk_with_import_origin(
+        &mut self,
+        kind: ThunkKind,
+        span: Span,
+        origin: Option<ImportedValueOrigin>,
+    ) -> ThunkId {
         let id = ThunkId(self.thunks.len() as u32);
         self.thunks.push(Thunk {
             kind,
             state: ThunkState::Unevaluated,
             span,
         });
+        self.thunk_import_origins.push(origin);
         id
     }
 
@@ -1269,6 +1436,10 @@ impl<L: ImportLoader> Engine<L> {
         self.thunks[thunk.0 as usize].span
     }
 
+    fn thunk_import_origin(&self, thunk: ThunkId) -> Option<&ImportedValueOrigin> {
+        self.thunk_import_origins[thunk.0 as usize].as_ref()
+    }
+
     fn with_path_context(
         &self,
         diagnostic: Diagnostic,
@@ -1327,6 +1498,9 @@ pub fn format_diagnostic_with<'a>(
             label.message,
         ));
     }
+    for note in &diagnostic.notes {
+        out.push_str(&format!("\nnote: {note}"));
+    }
     out
 }
 
@@ -1401,6 +1575,45 @@ fn format_path(path: &[String]) -> String {
     output
 }
 
+fn annotate_imported_value(
+    diagnostic: Diagnostic,
+    origin: Option<&ImportedValueOrigin>,
+    value: Option<&RuntimeValue>,
+) -> Diagnostic {
+    let Some(origin) = origin else {
+        return diagnostic;
+    };
+    if diagnostic
+        .notes
+        .iter()
+        .any(|note| note.starts_with("imported `"))
+    {
+        return diagnostic;
+    }
+    let path = if origin.path.is_empty() {
+        String::from("<root>")
+    } else {
+        format_path(&origin.path)
+    };
+    let value = value
+        .map(|value| format!(" supplied a value of type {}", runtime_value_kind(value)))
+        .unwrap_or_default();
+    diagnostic.with_note(format!("imported `{}` at `{path}`{value}", origin.key))
+}
+
+fn runtime_value_kind(value: &RuntimeValue) -> &'static str {
+    match value {
+        RuntimeValue::Concrete(ConcreteValue::String(_)) => "String",
+        RuntimeValue::Concrete(ConcreteValue::Int(_)) => "Int",
+        RuntimeValue::Concrete(ConcreteValue::Float(_)) => "Float",
+        RuntimeValue::Concrete(ConcreteValue::Bool(_)) => "Bool",
+        RuntimeValue::Concrete(ConcreteValue::Array(_)) => "Array",
+        RuntimeValue::Concrete(ConcreteValue::Object(_)) => "Object",
+        RuntimeValue::Concrete(ConcreteValue::Function(_)) => "Function",
+        RuntimeValue::Abstract(_) => "abstract",
+    }
+}
+
 #[cfg(feature = "regex")]
 fn satisfies_regex(value: &RuntimeValue, pattern: &str, span: Span) -> Result<()> {
     let RuntimeValue::Concrete(ConcreteValue::String(value)) = value else {
@@ -2127,6 +2340,86 @@ mod tests {
         );
     }
 
+    #[test]
+    fn structured_import_violation_reports_key_path_and_schema_span() {
+        let mut engine = Engine::new(MarkdownLoader {
+            source: String::from("---\ntitle: \"Hello\"\ndraft: maybe\n---\n# Hello"),
+        });
+        let module = engine
+            .add_root_source(
+                "main.dcdl",
+                "main.dcdl",
+                r#"
+                let
+                    Post = {
+                        frontmatter = {
+                            title = String;
+                            draft = Bool;
+                        };
+                        body = String;
+                    };
+                in Post & import "./post.md"
+                "#,
+            )
+            .unwrap();
+        let error = engine.eval_module(module).unwrap_err();
+
+        assert_eq!(error.kind, DiagnosticKind::ConstraintViolation);
+        assert_eq!(error.message, "expected Bool, found String");
+        assert!(
+            error
+                .labels
+                .iter()
+                .all(|label| label.span != Span::default())
+        );
+        assert!(
+            error
+                .labels
+                .iter()
+                .any(|label| label.message == "constraint declared here")
+        );
+        assert_eq!(
+            error.notes,
+            ["imported `content/post.md` at `frontmatter.draft` supplied a value of type String"]
+        );
+        assert!(engine.format_diagnostic(&error).contains(
+            "note: imported `content/post.md` at `frontmatter.draft` supplied a value of type String"
+        ));
+    }
+
+    struct StructuredArrayLoader;
+
+    impl ImportLoader for StructuredArrayLoader {
+        fn load(&mut self, _current_key: Option<&str>, _specifier: &str) -> Result<LoadedImport> {
+            Ok(LoadedImport::value(
+                "content/navigation.json",
+                HostValue::object([(
+                    "items",
+                    HostValue::array([HostValue::string("home"), HostValue::int(2)]),
+                )]),
+            ))
+        }
+    }
+
+    #[test]
+    fn structured_array_violation_reports_indexed_import_path() {
+        let mut engine = Engine::new(StructuredArrayLoader);
+        let module = engine
+            .add_root_source(
+                "main.dcdl",
+                "main.dcdl",
+                r#"{ items = [...String]; } & import "./navigation.json""#,
+            )
+            .unwrap();
+        let error = engine.eval_module(module).unwrap_err();
+
+        assert_eq!(error.message, "expected String, found Int");
+        assert_eq!(
+            error.notes,
+            ["imported `content/navigation.json` at `items[1]` supplied a value of type Int"]
+        );
+    }
+
     struct StructuredAliasLoader;
 
     impl ImportLoader for StructuredAliasLoader {
diff --git a/doc/manual/souce/design/diagnostics-and-fallback.md b/doc/manual/souce/design/diagnostics-and-fallback.md
index 871bbf1..5cb0f45 100644
--- a/doc/manual/souce/design/diagnostics-and-fallback.md
+++ b/doc/manual/souce/design/diagnostics-and-fallback.md
@@ -11,6 +11,7 @@ Diagnostic {
   span: Span
   message: String
   labels: Vec<DiagnosticLabel>
+  notes: Vec<String>
 }
 
 DiagnosticLabel {
@@ -22,6 +23,7 @@ DiagnosticLabel {
 `span` は primary location を示す。
 表示時には `Span.source` を source id のまま出すのではなく、可能な限り file path や virtual file name に解決する。
 `labels` は同じ error に関係する追加 location を示す。
+`notes` は source location を持たない semantic context を示す。
 合成や materialize の失敗では、衝突した constraint、value、default、または処理中 field path を label に含める。
 
 代表的な diagnostic kind:
@@ -37,6 +39,9 @@ DiagnosticLabel {
 - match failure
 - materialization failure
 
+Structured imports use semantic provenance rather than synthetic source spans.
+Constraint failures report the imported value's stable key and logical field or array path alongside the source span of the Decodal constraint that rejected it.
+
 ## エラーは値ではない
 
 評価失敗は `RuntimeValue` ではなく `Diagnostic` を返す。
diff --git a/doc/manual/souce/design/embedding-api.md b/doc/manual/souce/design/embedding-api.md
index 5028ceb..15a17c6 100644
--- a/doc/manual/souce/design/embedding-api.md
+++ b/doc/manual/souce/design/embedding-api.md
@@ -159,4 +159,7 @@ The core does not select content types or bundle Markdown/frontmatter parsers.
 The loader owns path resolution, media or extension dispatch, parsing rules, and parse diagnostics.
 The stable loader key is also used to cache structured imports.
 
+When a structured value fails a Decodal constraint, the diagnostic keeps the Decodal constraint span and identifies the host value by its stable import key and logical value path, such as `content/post.md` and `frontmatter.draft`.
+`HostValue` does not need source spans: syntax diagnostics for the external format remain the loader's responsibility, while cross-value validation reports semantic provenance.
+
 `load` is the single import hook: loaders dispatch by extension, media type, or another host-defined rule and return the appropriate variant directly.
diff --git a/packages/decodal-codemirror/wasm/decodal_language_tools_bg.wasm b/packages/decodal-codemirror/wasm/decodal_language_tools_bg.wasm
index d178fda..26fdfc2 100644
Binary files a/packages/decodal-codemirror/wasm/decodal_language_tools_bg.wasm and b/packages/decodal-codemirror/wasm/decodal_language_tools_bg.wasm differ
diff --git a/packages/decodal-wasm/decodal_wasm_bg.wasm b/packages/decodal-wasm/decodal_wasm_bg.wasm
index 8139ed9..b49e380 100644
Binary files a/packages/decodal-wasm/decodal_wasm_bg.wasm and b/packages/decodal-wasm/decodal_wasm_bg.wasm differ